I just finished the rear speaker install on my 98 986. I also changed the radio from the CDR-210 to the CDR-220 as well as the front speakers.
I used Polk DB-351s in the back (1 each side) and DB-401s in the front.
Nothing remarkable to report about the install except a few details.
I made speaker mounts out of 1/8" plywood from the hobby store, cut out with a dremel tool.
I made the grills from decorative aluminum sheet I found at Hobby Lobby. I trimmed it to fit, painted them black (along with the plywood). I held it all together with black E6000 glue. It came out great.
The hardest part was running the wires, along the ride side per the 101 projects book.
I did find the correct connector pins to go into the existing brown connector that plugs into the radio. Search for "ES#470511" on ECS tuning. They are only $0.99 each. By the way, ES#470510 is the same pin, but for smaller wire .
The pins are easy to crimp with a "Molex Open Barrelled Pin Crimper" like
this.
Just wanted to put the part numbers out there for someone else.
BTW - the system sounds 1000% better than the stock paper speakers and CDR-210. I was going to do door speakers and an amp but I think this is good enough for me. I love the look of the CDR-220. Very retro Porsche. I like watching it light up "Porsche" when it turns on.